This invention relates to a field effect transistor (herebelow referred to as a FET) employing an organic compound as a semiconductor.
Conventional FET's employ inorganic semiconductors such as monocrystal silicon or GaAs. However, FET's using inorganic semiconductors are disadvantageous in that they are expensive and in many cases are complicated to manufacture. Furthermore, there are limitations on the size of an inorganic semiconductor device.
FET's are frequently used as active drive elements for large area liquid crystal displays. At present, such FET's are typically thin film transistors using amorphous silicon formed by CVD (chemical vapor deposition). As the area of the display increases, it becomes difficult and expensive to manufacture a large number of uniform inorganic FET's on a single substrate. Therefore, attempts have recently been made to manufacture a FET using an organic semiconductor. Of organic semiconductors, those comprising a .pi.-conjugated polymer have excellent semiconductor properties. Moreover, .pi.-conjugated polymers have the excellent formability which is characteristic of polymers, so they can be readily formed into semiconductor layers having a large area. This has made them particularly attractive for use in FET's.
Various .pi.-conjugated polymers have been used as semiconductors in FET's, including polyacetylene (see Journal of Applied Physics, Vol. 54, p. 3255 (1985)), poly(N-methylpyrrole) (see Chemistry Letters, p. 863 (1986)), polythiophene (see Applied Physics Letters, Vol. 49, pp. 1210-1212 (1986)), and poly(3-hexylthiophene) (see Applied Physics Letters, Vol. 53, p. 195 (1988)). However, none of these materials has been found completely satisfactory for practical applications. In a FET employing polyacetylene as a semiconductor, the source-drain current which can be generated in response to the application of a gate voltage is extremely small. A FET element using polythiophene as a semiconductor has a much higher source-drain current which can be modulated by 100-1000 times by controlling the gate voltage. However, the ON/OFF ratio and the source-drain current of a FET employing polythiophene as a semiconductor are still relatively low compared to those of a FET employing an inorganic semiconductor. Furthermore, as polythiophene is normally formed by electrochemical polymerization, it is difficult to simultaneously form a large number of uniform FET's using polythiophene. Although poly(3-hexylthiophene) has the advantage that it is soluble in various solvents, a FET employing it as a semiconductor does not have properties, such as source-drain current and ON/OFF ratio, as good as those of a FET employing polythiophene. Poly(N-methylpyrrole) is also not completely satisfactory as an organic semiconductor for a FET.
Thus, a conventional FET using an organic semiconductor has the problem that the magnitude of the source-drain current and the extent to which it can be modulated by a gate voltage are low.
It is possible to increase the source-drain current of a FET using a .pi.-conjugated polymer as a semiconductor by doping the .pi.-conjugated polymer. However, as doping increases the electrical conductivity of the entire semiconductor channel, the source-drain current of the resulting FET increases when no gate voltage is applied (the leak current). As a result, the modulation width of the source-drain current in response to a gate voltage is small, and the properties such as source-drain current and ON/OFF ration, of the FET are poor.